In SMTP, must the RCPT TO: and TO: match?

No, they don’t have to match. When the message is sent, the SMTP Server (aka Message Transfer Agent or MTA) is creating a so called SMTP envelope which contains the recipients and the sender of the message (see RFC5321):

SMTP transports a mail object. A mail object contains an envelope and content. The SMTP envelope is sent as a series of SMTP protocol
units (described in Section 3). It consists of an originator
address (to which error reports should be directed), one or more
recipient addresses, and optional protocol extension material.

It is, actually, quite often that the RCPT TO: Command has more recipients that the header of the message – one common case is the usage of “blind copies” bcc: (see RFC5321):

Addresses that do not appear in the message header section may appear
in the RCPT commands to an SMTP server for a number of reasons. The
two most common involve the use of a mailing address as a “list
exploder” (a single address that resolves into multiple addresses) and
the appearance of “blind copies”.

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